1. Field of the Invention
The present invention relates to a track searching method, and more particularly, to a method for determining a track searching direction of an optical pick-up head.
2. Description of Related Art
FIG. 1 is a diagram showing a conventional track searching method. As shown in FIG. 1, the blank portions are tracks (or lands) of an optical disc and the dotted portions are grooves of the optical disc. In addition, the illuminated area on the optical disc produced by the optical pick-up head can be divided into four blocks A, B, C and D according to their locations. Tracking error can be calculated using the formula TE=(A+D)−(B+C). When the photo-diode (PD) in the optical pick-up head moves into the middle of a track, the tracking error signal is equal to a particular reference voltage. When the photo-diode moves to the right, because the area B and the area C have moved into a darker groove, the value of (A+D)−(B+C) starts to increase. On the contrary, when the photo-diode moves to the left, the area A and the area D have moved into a darker groove so that the value of (A+D)−(B+C) starts to decrease.
On the other hand, the conventional method of calculating the radio frequency ripple (RFRP) value is RFRP=(A+B+C+D). When the photo-diode is in the middle of a track, the RFRP value is the largest. When the photo-diode moves either to the left or to the right, the RFRP value starts to decrease. Therefore, when the RFRP value leads the TE signal by a phase angle of 90°, the track searching direction is determined to be forward. Conversely, when the TE signal leads the RFRP value by a phase angle of 90°, the track searching direction is determined to be backward (as shown in FIG. 2).
The foregoing method of determining a track searching direction has been widely adopted for reading data from optical disc. However, when the same method is applied to a recordable optical disc system, track searching has to be carried out on a blank optical disc. Because the grooves of the optical disc have no written data (marks or pits), the brightness/darkness contrast between the lands and the grooves is rather small. Hence, the signal-to-noise ratio of the RFRP signal is rather low and the phase difference between the RFRP and the TE signal cannot be contrasted reliably.